A zinc-lysine complex (“ZLC”) having the chemical structure [Zn(C6H14N2O2)2Cl]+Cl−, has recently been described. See, e.g. International Patent Application Nos. PCT/US2012/70489 and PCT/US2012/70498, filed on 19 Dec. 2012, incorporated by reference in its entirety. ZLC has the unusual property that under conditions of increasing dilution rather than going into or remaining in solution as the solution becomes more dilute, as would typically be the case for an ionic complex, the ZLC hydrolyzes, to provide a relatively insoluble zinc oxide precipitate. The ZLC is useful in antiperspirant products, as it precipitates under sweat conditions that can plug the pores and block sweat release. In the presence of protein, the ZLC will flocculate and plug the sweat glands. The zinc oxide precipitate can also inhibit odor-causing bacteria, making ZLC useful as a deodorant. ZLC is also useful in skin care products, for example liquid hand soap or body wash, providing controlled deposition of zinc oxide on the skin upon dilution and use, which has an antibacterial effect, and also may help protect against sun damage. Finally ZLC can be incorporated in oral care formulations, which upon use and dilution, provide a zinc oxide precipitate that is effective for inhibiting and treating dentinal hypersensitivity, dental caries, and enamel erosion and demineralization.
Prior syntheses of ZLC involved adding solid ZnO to a solution of lysine.HCl in water under near-neutral conditions; stirring for 12 hours; centrifugation to remove unreacted solids, and precipitation of ZLC by pouring the solution into ethanol. This procedure possess several disadvantages, including long reaction time, and extra steps for removal of the unreacted insoluble reagents, which results in excess amino acid in the final reaction mixture hindering product purity, in turn requiring further isolation with ethyl alcohol, and thus limiting feasibility on a large manufacturing scale. Thus, it can be seen that there is a need for improved methods for producing ZLC.